If .mp3 is in your ASCII-list, there's no wonder you're having difficulties. When using auto-mode, every extension in the ASCII-list gets transfered in ASCII-mode, ie. as text. You wouldn't want that for mp3-files, so the first thing you should do is remove the .mp3-extension from that list.

I'm ofcourse assuming you're not using a cracked version, because that would also be cause for erratic behaviour.